The present applicant has progressively developed delivery devices for covered stents. One example is to be found in WO 2005/053574, which is incorporated by reference in its entirety into this application. The present invention carries such architecture forward into the technical field of covered stents that are lengthier (maybe up to 120 mm, or even beyond) and so put greater demands on the strength of the delivery system during deployment of the stent, when a sheath surrounding the stent has to be pulled proximally, relative to the stent, to release the stent into the bodily lumen to be stented, progressively, starting at the distal end of covered stent. Readers will appreciate that it is characteristic of such catheter delivery devices that one of two end-to-end components is in endwise tension (the components responsible for pulling the sheath proximally) while the other of the two end-to-end components (the one that prevents the stent from being drawn proximally with the proximally moving sheath) is in end-to-end compression during progressive deployment of the stent. In general, designers of catheter delivery devices for self-expanding stents try to keep the passing diameter of the catheter to a minimum, which self-evidently conflicts with the design imperative that the end-to-end component in compression does not buckle or concertina, or otherwise lose its length integrity, during stent deployment. The present invention sacrifices ultimate narrowness of passing diameter to the objective of enhanced performance in delivering relatively lengthy covered stents, for which endwise stresses in the catheter delivery device, during deployment of the stent, are likely to be higher than for shorter stents that are bare rather than covered.
Catheter delivery devices that exhibit a “pull wire” are attractive for tasks where the end-to-end stresses are relatively large, because a wire is well able to sustain endwise tension, but not so well adapted to sustain an endwise compressive stress. Better for that task is a tube. Thus, in a coaxial system, one with the tube carrying the compressive stress, and the wire within it carrying the tensile stress, will likely perform better than a system arranged the other way around. However, at the distal end of the system, where the stent is located, it is the outside sheath that is in tension, and the inside catheter element, that stops the stent moving proximally, that is in compression. Thus, somewhere between the proximal and distal end of the catheter delivery system of a pull wire architecture, there needs to be an inversion, to transfer the tensile stress from the radially inner pull wire to the radially outer sheath surrounding the stent. Clearly, any length interval between the transfer zone and the distal end of the stent, in which a relatively small diameter component of the catheter system is required to carry the endwise compressive stress during stent deployment, needs to be strong enough to retain its lengthwise integrity during such stent deployment. Self-evidently, the length of that portion should be reduced, to the extent possible.
A further problem with transfer zones in pull wire systems is to minimize any propensity for the catheter system to buckle at any particular point along its length. Evidently, there is a challenge to incorporate a transfer zone somewhere along the length of the catheter shaft, while at the same time avoiding any points long the length of the catheter where there is an increased risk of buckling.